The present invention relates to a battery charging device for charging batteries and a charging method thereof, and particularly to a battery charging device for charging batteries and a charging method thereof that may be favorably employed for charging batteries that generate a large amount of heat at the time of charging such as nickel-metal hydride batteries.
Rechargeable batteries that may be repeatedly used are currently being employed as power sources for power tools. Nickel-cadmium batteries are being widely used as batteries in such power tools, and battery charging devices performing rapid charge by supplying a large amount of current to the batteries are being employed. More particularly, by performing rapid charge of batteries in approximately 20 minutes, successive use of power tools is made possible while exchanging run down batteries with charged ones.
The inventors have considered whether it would be possible to improve performance of power tools by using nickel-metal hydride batteries as batteries. However, while nickel-metal hydride batteries enable an increase in capacity compared to nickel-cadmium batteries, they exhibit drawbacks in that the amount of generated heat at the time of charge is large, and in case a high temperature is reached through the heat generation, electrodes of internal cells of batteries or separators would degrade to thereby shorten the life of the batteries. Thus, rapid charge through a large amount of current as it had been performed in the above-described nickel-cadmium batteries could not be performed.
The invention has been made for solving the above-described subjects, and it is an object thereof to provide a battery charging device and a charging method thereof with which batteries may be charged in a short time while restricting rises in temperature of the batteries and without performing excessive charge.